The present invention relates to an objective optical element used for an optical pick-up apparatus by which the recording/reproducing of an optical information recording medium whose operating wavelength is different and transparent substrate thickness is different is conducted by one objective lens, and optical pick-up apparatus and optical information recording reproducing apparatus, and particularly to the objective optical element in which a light source portion which oscillates the different wavelength is used for the modularized light source (2-laser 1 package module), or used for an optical system in which the focal length of the objective lens is short and which is sensitive to the error factors, and in which the image height coma characteristic at the time when each optical information recording medium is used, is improved, and the optical pick-up apparatus using it, and the optical information recording reproducing apparatus.
Nowadays, many kinds of optical information recording media exist, and the regulations for these optical information recording media are determined as shown in [Table 1]. In this connection, after this (the lens data in Table is included), it is defined that the exponent of 10 (for example, 2.5×10−3) is expressed by using E (for example, 2.5×E-3).
TABLE 1TransparentNecessary numericalsubstrateaperture NAthickness(light sourceOptical disk(mm)wavelength λ nm)CD, CD-R (reproducing)1.200.45 (λ = 780 nm)CD-R (recording, reproducing)1.200.50 (λ = 780 nm)DVD0.600.60 (λ = 635 nm)
Herein, as the media for which the interchangeability of mutual optical information recording media whose recording density is different is required, there are DVD and CD. In these optical information recording media, as shown in [Table 1], respectively, each transparent substrate thickness is different. In order to secure the interchangeability, it is necessary that the spherical aberration generated by the difference of this transparent substrate thickness is corrected by any means. Further, in DVD and CD, because the required numerical aperture is different, any countermeasure is necessary also for this.
In order to realize the optical pick-up apparatus having the interchangeability of the DVD/CD, the objective lens on which the diffractive structure is provided, is developed. As such an objective lens, for example, on the one side surface of the objective lens, the diffractive structure which is different at the inside or outside of the specific height h from the optical axis is provided, and in the inside region, the spherical aberration is corrected for each transparent substrate thickness, and in the outside region, the spherical aberration is corrected only for the DVD, and there is a diffractive structure in which the spherical aberration is not corrected for the CD, but is made flare. When the objective lens is structured in this manner, on each of optical information recording media, the converging spot which is respectively required at the time of the recording or reproducing of the information can be adequately formed.
In this connection, when such a diffractive structure is used for the objective lens, the correction of the spherical aberration when the information is recorded or reproduced on both of DVD/CD can be comparatively easily conducted. However, as the optical characteristic deterioration factor to obstacle the recording or reproducing of the adequate information, the coma also exists other than the spherical aberration. In the case where the coma is large, when the light flux tilted to the optical axis is incident, by the inclination of the objective lens generated due to the assembling error, there is a possibility that the formation of the adequate spot on the optical information recording medium is obstructed. However, when the above-described diffractive structure is used, both of the spherical aberrations at the time when both of the DVD/CD are used, can be reduced, but for the coma, there is a problem that the both can not be simultaneously corrected.
Particularly, as the light source of the optical pick-up apparatus to attain the interchangeability of the DVD/CD, a light source in which 2 semiconductor lasers called so-called “2-laser 1 package”, are assembled on one substrate and are made as one unit, is well known. When such a light source is used, and the recording or reproducing of the information is conducted for both of the DVD/CD, for example, when the light source for the DVD is arranged on the optical axis of the objective lens, the light source for the CD is arranged at the position dislocated from the optical axis without fail, and accordingly, in such a case, it can be said that it is preferable that the coma when the CD is used, is reduced as much as possible.